Disallow changing discriminator key in update unless overwriteDiscriminatorKey option set

[vkarpov15]

Re: #6087, now that we have an option to explicitly allow changing the discriminator key, we should disallow setting the discriminator key in updateOne(), etc. unless that option is set.

[vkarpov15]

Here's a script that should succeed in 6.0:

const assert = require('assert');
const mongoose = require('mongoose');

mongoose.set('debug', true);
mongoose.set('useFindAndModify', false);

const { Schema } = mongoose;

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });

  await mongoose.connection.dropDatabase();

  const baseSchema = new Schema({}, { discriminatorKey: 'type' });
  const baseModel = mongoose.model('thing', baseSchema);

  const aSchema = new Schema(
    {
      aThing: { type: Number },
    },
    { _id: false, id: false },
  );
  const aModel = baseModel.discriminator('A', aSchema);

  const bSchema = new Schema(
    {
      bThing: { type: String },
    },
    { _id: false, id: false },
  );
  const bModel = baseModel.discriminator('B', bSchema);

  // Model is created as a type A
  const doc = await baseModel.create({ type: 'A', aThing: 1 });

  const res = await baseModel.findByIdAndUpdate(
    doc._id,
    { type: 'A', bThing: 'one', aThing: '2' },
    { runValidators: true, /*overwriteDiscriminatorKey: true*/ },
  );
  assert.equal(res.type, 'B');
}